One of conventional devices using displacement generated by a piezoelectric effect as a mechanical driving source is an actuator. Specifically, compared with an electromagnetic actuator, a multilayer actuator in which a piezoelectric layer and an internal electrode are laminated has lower power consumption, a lower heating value and better responsivity, and the size and weight thereof can be reduced. Therefore, in recent years, the multilayer actuator is used in various fields such as a needle selection control of a fabric-knitting machine.
Piezoelectric ceramics used for these actuators are required to have larger piezoelectric properties, specifically a larger piezoelectric strain constant. As a piezoelectric ceramic capable of obtaining a larger piezoelectric strain constant, for example, a ternary system of lead titanate (PbTiO3; PT), lead zirconate (PbZrO3; PZ) and lead zinc niobate (Pb(Zn1/3Nb2/3)O3) is known (refer to Japanese Examined Patent Application Publication No. Sho 44-17344). Further, as an improved ternary system, part of lead (Pb) is replaced with barium (Ba), strontium (Sr), calcium (Ca) or the like to improve a dielectric constant and a mechanical coupling factor (refer to Japanese Examined Patent Application Publication No. Sho 45-39977 and Japanese Unexamined Patent Application Publication No. Sho 61-129888), or the contents of lead, barium and strontium are limited within a specific range to improve variations in properties between products, thereby resulting in an improved piezoelectric strain constant (refer to Japanese Unexamined Patent Application Publication No. Hei 3-256379). Moreover, a quaternary system including lead magnesium niobate (Pb(Mg1/3Nb2/3)O3) in addition to the ternary system has been reported (refer to Japanese Unexamined Patent Application Publication No. Hei 8-151264).
However, the firing temperature of a conventional piezoelectric ceramic is as high as 1200° C. or over, so when the piezoelectric layer and the internal electrode are laminated like the multilayer actuator, and then fired, a high-cost noble metal capable of resisting a high temperature of 1200° C. or over such as platinum (Pt) or palladium (Pd) must be used, which causes a problem in its manufacturing cost. Further, in order to reduce the firing temperature, a step of forming powders with a large specific surface area after pre-firing, or a step of applying pressure during firing must be carried out, which causes its manufacturing process to be complicated.
As a less expensive material of the internal electrode, a silver-palladium alloy (Ag—Pd alloy) is known. However, when the palladium content exceeds 30 mass %, palladium induces a reduction reaction during firing, which may cause a defect such as the occurrence of a crack or peeling of the electrode. Therefore, the palladium content is preferably 30 mass % or less. In order to contain 30 mass % or less of palladium, according to the phase diagram of the silver-palladium system, the firing temperature is required to be 1150° C. or less, preferably 1120° C. or less. Moreover, in order to reduce the manufacturing cost, it is necessary to further reduce the palladium content. For that purpose, the firing temperature is required to be as low as possible. For example, in order to contain 20 mass % or less of palladium, the firing temperature is required to be 1050° C. or less, preferably 1000° C. or less.
Moreover, in recent years, the development of smaller size and lower profile actuators, such as a study of the use of much smaller size actuators than the conventional one called micro-actuators for controlling the positioning of hard disk heads, have been increasingly advanced.
However, as the displacement of the actuator using the piezoelectric ceramic is proportional to the volume thereof, when the size of the actuator is simply reduced, the displacement thereof is also reduced. In order to reduce the size of the actuator without reducing the displacement, it is required to increase the number of layers to laminate or to use a piezoelectric ceramic having a larger piezoelectric strain constant. However, when the number of layers increases, the amount of the material required for the internal electrode and the number of manufacturing steps increase, thereby resulting in increased cost, and further thinner layers must be achieved, so it is not preferable. Therefore, a piezoelectric ceramic having a further larger piezoelectric strain constant is required. For example, the piezoelectric strain constant d33 in a thickness longitudinal vibration mode is 550 pC/N or over, and preferably 600 pC/N or over.
In addition, as advances in size and profile of a piezoelectric device make the mechanical strength of the device decline, the device is prone to be damaged during manufacturing and using, and to lead a decline in yield and deterioration in properties. Therefore, a larger mechanical strength is required.
Moreover, the piezoelectric ceramic for the actuator is required to have a larger piezoelectric strain constant as well as a higher Curie temperature and to suffer a smaller deterioration in piezoelectric properties with time. For example, in recent years, actuators for driving a hard disk head of a personal computer or the like used at a high temperature of approximately 100° C. or 150° C. have been increasing, so piezoelectric ceramics applicable to such actuators are desired to have a Curie temperature of 200° C. or over, preferably 300° C. or over. Further, in recent years, the actuators are more often used to control more accurate positioning than before, so a smaller deterioration in piezoelectric properties with time is required. In such a case, the piezoelectric strain constant d31 in a rectangular extensional vibration mode is preferably 200 pC/N or over. In the conventional piezoelectric ceramic, a larger piezoelectric strain constant can be obtained, but there is a problem that the Curie temperature is low or the deterioration in piezoelectric properties with time is large.
In view of the foregoing, it is an object of the present invention to provide a piezoelectric ceramic having superior properties such as a higher piezoelectric strain constant and capable of being fired at a lower temperature, a method of manufacturing the same, and a piezoelectric device.